The present invention relates to a receiver which conducts, for example, Fast Fourier Transform (FFT) for a received signal to demodulate a carrier signal contained in the received signal, and in particular, to a receiver capable of improving quality of signal reception even if interference occurs between carriers.
For example, an Orthogonal Frequency Division Multiplexing (OFDM) scheme is employed as a communication scheme in a mobile communication system and the like.
FIG. 5 shows an example of a configuration of an OFDM receiver to receive an OFDM signal which is modulated by a transmission side (an OFDM transmitter) using an OFDM scheme and which is transmitted by radio therefrom.
In FIG. 5, the similar processing sections as those of FIGS. 1 and 4, which will be referred to for embodiments later, are assigned with the same reference numerals. However, this does not intend to restrict the present invention.
In the OFDM receiver of this example, the OFDM signal sent from the transmission side is received by an antenna 1, the received signal is processed by a Radio Frequency (RF) section 2, and the processed signal is then converted by an Analog to Digital (A/D) converter 3 from an analog signal into a digital signal at sampling timing of n MHz. The digital signal is converted by a Serial to Parallel (S/P) converter 31 from a serial signal into a parallel signal. The parallel signal is inputted to a guard interval remover 32 and an FFT section 33.
In the guard interval remover 32, a guard interval signal portion is removed from the parallel signal. In the FFT section 33, N-point FFT processing is conducted for a signal portion of data to produce N FFT results. The signals of N FFT processing results are demodulated by demodulators A(1) to A(N) respectively associated therewith. The N demodulation resultant signals constituting a parallel signal are converted by a Parallel to Serial (P/S) converter 8 from a parallel signal into a serial signal.
As the value of n (sampling frequency) for the sampling timing of the A/D converter 3, there is used an arbitrary value (e.g., the value of a sampling rate required in the OFDM scheme). As the value of N for the FFT processing in the FFT section 33, there is employed, for example, a value equal to or slightly less than the number of carriers (subcarriers) contained in the OFDM signal.
It has been known that the OFDM scheme is resistive against multi-path interference. On the other hand, there exists a problem that the OFDM scheme is prone to be influenced by the shift in the frequency (e.g., the value associated with the Automatic Frequency Control (AFC)), the Doppler shift, and the jitter that are caused when the OFDM scheme is used in mobile devices.
FIG. 6 shows an example of a waveform of the OFDM signal with distortion in which the abscissa represents the frequency.
Such distortion is caused due to the narrow carrier interval in the OFDM scheme, specifically, by the Inter Carrier Interference (ICI) for the variation with respect to the frequency axis such as the frequency shift, the Doppler shift, and the jitter. Particularly, for a high-speed mobile unit, it is essential to solve the problem of ICI caused by the Doppler shift.
The OFDM scheme is strong against the multi-path interference because of presence of the guard interval. However, the orthogonality is disturbed by the multi-path, causing the frequency-selective fading. As a result, although partially, there inevitably takes place the problem of ICI.
Reference may be made to JP-A-2002-261720.